This invention relates to shoe manufacturing and, particularly, to improvements in reinforcing and stiffening portions of a shoe such as a shank region of the insole which extends from the heel to the ball portion. The invention relates to improvements in articles for forming a shank stiffener of the type disclosed in U.S. patent application Ser. No. 681,562, filed jointly by me with D. Bray and R. Peterson on Apr. 29, 1976 and which is assigned to the asignee of this application. That application discloses techniques and articles for forming shoe shanks, the articles being in the form of an elongate shank strip or rope having a carrier sleeve which contains a plurality of fiberglass strands in a thermosetting plastic resin matrix. The thermosetting matrix is activatable in response to a selected external stimulus such as, for example, radiant heat. The sleeve preferably is formed from a material which is transparent to radiant energy to permit activation of the resin in situ on the insole bottom. The shank thus formed adheres to the insole bottom by any of a variety of means including but not limited to, melting of the sleeve to form an adhesive bond, direct contact between the resin matrix and the insole, application of an adhesive layer between the shank strip and the insole or a combination of these. Although the articles and techniques disclosed in said application have proved to be effective in accomplishing their objectives, there may be some instances in which the resin tends to expand as it cures, which may result in less than desired uniformity in the shape of the shanks. While in most instances, the expansion of the resin may be controlled or avoided by carefully controlling the conditions under which the shank strip is heated and cured, the use of fine or sensitive control procedures preferably is to be avoided under production conditions. The present invention employs an improved sleeve structure and method for automatically controlling the shape of the stiffener. The present invention reduces considerably the need for external controls and, in addition, provides an improved means by which the shank strip may be handled.
In brief, the present invention resides in the use of a carrier sleeve having upper and lower surfaces formed from separate sheets or strips which may be of different materials. The strips are attached to each other along their longitudinal edges which define relatively wide margins. The fiberglass strands and thermosetting matrix extend longitudinally within the middle of the sleeve, between the margins. The upper strip or surface of the sleeve preferably is substantially transparent to the radiant energy to permit the resin to be activated. The upper strip preferably is formed from a material which will not melt, deteriorate or otherwise lose its strength (for example, its tensile properties) from exposure to the radiant heat or from exothermal heat generated during the curing process, or at least until the resin has assumed a substantially final shape. The lower, insole-engaging surface of the sleeve preferably is thermoplastic and preferably will melt under the influence of the applied and/or exothermally generated heat to serve as an adhesive bond between the cured shank strip and the insole bottom. The wide margins of the carrier sleeve provide a means by which the shank strip may be held against the insole bottom to retain the carrier sleeve in place during the activation and curing process. During activation and curing, the tendency for the matrix to expand (for example, because of formation of gas bubbles) is resisted by the upper surface of the carrier sleeve which serves to confine the resin (and the bottom thermoplastic strip) between it and the insole bottom. This is effective to preclude the strip from assuming a freely expanded shape which, in some types of shoes, is undesirable. In a variant of the invention, the upper strip of the carrier sleeve is formed from a shrinkable film which will shrink during activation and curing of the resin. As the upper strip shrinks, it causes the resin matrix to be pressed into a cross sectional shape having a reduced height and smoothly tapered edges.
It is among the general objects of the invention to provide an improved sleeve construction for use in elongate stiffening devices of the type described.
Another object of the invention is to provide an improved article for use in connection with stiffening of shoe insoles or the like in which the article includes self-contained means by which the shape of the cured stiffener may be self-controlled.
A further object of the invention is to provide an improved elongate shank stiffener of the type described having a curable resin surrounded by a carrier sleeve and in which the carrier sleeve has upper and lower surfaces formed from different materials.
Another object of the invention is to provide a shank strip having a carrier sleeve of the type described in which the upper surface of the carrier sleeve has a high melting temperature and will not lose its tensile strength as a result of heat applied to or generated by the thermosetting matrix during curing at least until the resin has cured to a substantially final shape.
A further object of the invention is to provide a shank strip of the type described in which the carrier sleeve has wide margins to facilitate handling of the shank strip.